Steel material for sliding members and method of manufacturing the same

ABSTRACT

A steel material for sliding members with improved seizure resistance is provided. A steel material for sliding members has a chemical composition including, in mass %, 0.05 to 5.0% In, and a microstructure with metal In particles or particles of oxides mainly composed of In dispersed therein, those ones of the metal In particles which have diameters of 50 nm to 5 μm being in a number density not lower than 5000 particles/mm2 or those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm being in a number density not lower than 30 particles/mm2.

TECHNICAL FIELD

The present invention relates to a steel material for sliding members and a method of manufacturing such a material.

BACKGROUND ART

Members required to have certain sliding properties include crankshafts for automobile engines. In recent years, there has been a growing trend toward smaller engines, lower lubricant viscosities and simpler lubrication systems to reduce fuel consumption; further, to improve dry-start performance during intermittent operations and for other purposes, steel materials for sliding members used in crankshafts are required to have even higher seizure resistance.

The occurrence of seizure on steel material is governed by physical factors such as abrasion powder produced during sliding and, in addition, chemical factors occurring at the sliding interfaces (for example, transfer from and adhesion of a member to another member over which that member slides, production of materials of different types that are thought to be caused by chemical reactions). In a dry environment, measures taken to prevent seizure include, for example, diamond-like carbon (DLC) film formation on the surface of steel material, and coating of steel material with a PTFE-based fluorine resin.

In the case of a crankshaft, seizure occurs at contacts between the crankshaft and the bearings. The crankshaft and bearings are usually in a state of fluid lubrication in which they slide over each other with lubricant being present therebetween. However, as sliding at high speed and high pressure raises the temperatures at the contact interfaces, the chemical quality of the lubricant may change. This may cause the lubricant to lose its performance or wettability, bringing the crankshaft and bearings in direct contact, which is thought to cause plastic deformation of the microstructure of the steel material, resulting in seizure.

In view of this, attempts have been made to increase the mechanical strength of steel material to reduce abrasion loss and/or plastic deformation of the steel material, or control the surface quality of steel material to improve its wettability with respect to lubricant. For example, JP Hei7(1995)-18379 A discloses a steel for mechanical structures including, at its surface, a compound layer produced by soft nitriding to provide good seizure resistance and fatigue strength. JP 2007-238965 A discloses a crankshaft including an induction-hardened layer at its surface to provide good seizure resistance.

Another approach that has been proposed is a sliding member that uses solid lubricant. For example, JP Sho60(1985)-1424 A discloses a sliding member including a base with a surface provided with recesses and protrusions, and solid lubricant held in the recesses of the base.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP Hei7(1995)-18379 A -   Patent Document 2: JP 2007-238965 A -   Patent Document 3: JP Sho60(1985)-1424 A -   Patent Document 4: JP 2001-131684 A

DISCLOSURE OF THE INVENTION

Providing the surface of a steel member with such a DLC coating as discussed above provides good surface characteristics. However, in a high-temperature, high-pressure environment during sliding, unexpected heat impact and stress concentration may occur. The reality is that a DLC coating cannot sufficiently resist such an environment. Further, a fluorine resin coating as discussed above, which represents a highly convenient and low-cost surface treatment, suffers from problems such as early corrosion degradation that is thought to be caused by pin holes formed inside the fluorine resin, and environmental concerns such as high likelihood to produce toxic, dangerous gases in a high-temperature environment. Thus, the above-discussed methods have limits to the ability to sufficiently prevent seizure on the surface of steel material and maintain sliding performance at good levels.

Further, the techniques described in Patent Documents 1 to 3 have the following problems.

Soft nitriding and carburization, discussed above, are methods for modifying the chemical quality of the surface of a steel member to provide a harder composition/microstructure, and include a gas treatment at high temperature, separate from the normal heat treatment process, which requires extra process time and increases equipment costs. Further, a steel member to be processed is restricted by the capacity inside the gas treatment furnace being used, a problem that is not negligible.

JP Sho60(1985)-1424 A teaches performing shot blasting and etching on the surface of the base to create recesses and protrusions, and embedding solid lubricant in these recesses. This method involves a complicated manufacturing process. Another concern is that the solid lubricant wears out at the sliding surface and, without further supply, the lubrication performance may be significantly impaired.

An object of the present invention is to provide a steel material for sliding members with improved seizure resistance and a method of manufacturing such a steel material.

A steel material for sliding members according to an embodiment of the present invention has a chemical composition including, in mass %, 0.05 to 5.0% In, and a microstructure with metal In particles or particles of oxides mainly composed of In dispersed therein, those ones of the metal In particles which have diameters of 50 nm to 5 μm being in a number density not lower than 5000 particles/mm² or those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm being in a number density not lower than 30 particles/mm².

In the steel material for sliding members according to an embodiment of the present invention, those ones of the metal In particles which have diameters of 50 nm to 5 μm is in a number density not lower than 5000 particles/mm².

A method of manufacturing a steel material for sliding members according to an embodiment of the present invention is a method of manufacturing the above-described steel material for sliding members, including: preparing raw material having a chemical composition including, in mass %, 0.05 to 5.0% In; and subjecting the raw material to a quenching process in which the material is heated at 800 to 1200° C. for 5 to 30 minutes and then water cooled or oil cooled.

In the steel material for sliding members according to an embodiment of the present invention, the chemical composition includes an In content of, in mass %, 0.3 to 5.0%, and those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm is in a number density not lower than 30 particles/mm².

A method of manufacturing a steel material for sliding members according to an embodiment of the present invention is a method of manufacturing the above-described steel material for sliding members, including: preparing raw material having a chemical composition including, in mass %, 0.3 to 5.0% In; subjecting the raw material to a quenching process in which the material is heated at 800 to 1200° C. for 5 to 30 minutes and then water cooled or oil cooled; and subjecting the quenched material to a heat treatment in which the material is heated at 150 to 650° C. for 5 to 60 minutes and then furnace cooled.

The present invention provides a steel material for sliding members with improved seizure resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing how, in friction testing in a dry environment, coefficient of friction changed over time for a test material with a maximum coefficient of friction exceeding 0.5 and a test material with a maximum coefficient of friction remaining 0.5 or lower.

FIG. 2 is a graph showing how, in friction testing in a wet environment, coefficient of friction changed over time for a test material with a maximum coefficient of friction exceeding 0.5 and a test material with a maximum coefficient of friction remaining 0.5 or lower.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

The present inventors focused on metal elements added to steel material and attempted to find a way of improving seizure resistance that is more convenient and more effective than conventional ways, and found out that adding indium (In) is effective to improve seizure resistance.

Indium has a low melting point (156° C.) compared with other metal elements, and is soft (with a Mohs' hardness of 1.2). Indium is used as a solid lubricant, like lead and graphite, which have similar characteristics (for example, see JP 2001-131684 A). However, solid lubricant wears off at the sliding surface and, without supply, its lubrication performance may be significantly impaired.

The present inventors discovered that if fine particles of metal In or fine particles of oxides mainly composed of In are dispersed in steel, a lubrication coating can remain on the sliding surface for a prolonged period of time.

Indium that has not dissolved in steel is present as precipitates. This undissolved In is diffused across the surface of the steel even in a normal-temperature environment and forms an In concentration layer, and this In concentration layer serves as a lubrication coating to contribute to improvements in sliding properties. Further, when the lubrication coating wears out due to sliding, undissolved In in the steel diffuses across the steel surface without delay to reproduce a lubrication coating.

The form of In precipitates varies depending on the heat treatment during manufacture. Specifically, if temperature during the cooling has passed through the range of 650 to 100° C. at a rate not lower than that for air cooling, the number of In particles that has single-precipitated (hereinafter “metal In”) is large. On the other hand, if the steel has been held in the above-mentioned temperature range for a prolonged period of time or temperature has passed through the above-mentioned range at a lower cooling rate, for example that for furnace cooling, the number of particles of In oxides is large.

To ensure that the lubrication coating is reproduced more quickly to keep good sliding properties for a prolonged period of time, fine In particles must be dispersed, and the number density of In particles required differs depending on the form of In precipitates. Specifically, good sliding properties can be obtained for a prolonged period of time if the number density of particles of metal In with diameters of 50 nm to 5 μm is not lower than 5000 particles/mm² or the number density of particles of oxides mainly composed of In with diameters not smaller than 1 μm is not lower than 30 particles/mm². As used herein, “diameter”, for non-spherical particles, means equivalent circle diameter as measured in a cut surface being observed.

The present invention was made based on the above-discussed findings. A steel material for sliding members according to an embodiment of the present invention will now be described in detail. In the following description, “%” for the content of an element means mass percent.

[Steel Material for Sliding Members]

[Chemical Composition]

The steel material for sliding members according to the present embodiment has a chemical composition including 0.05 to 5.0% In.

In: 0.05 to 5.0%

Indium (In) forms a lubrication coating on the surface of steel material to improve the sliding properties of the steel material. On the other hand, for an excessive In content, segregation at the grain boundaries is significant, potentially causing brittle fracture or corrosion along the grain boundaries. In view of this, the In content is to be 0.05 to 5.0%. A lower limit for In content is preferably 0.1%, more preferably 0.2%, and still more preferably 0.3%. An upper limit for In content is preferably 4.0%, and more preferably 3.0%.

If In is to precipitate as metal In, desired sliding properties can be provided even with a low In content, in comparison with cases where In precipitates as oxides. When manufacture costs are considered, the lower In content, the better. In view of this, if In is to precipitate as metal In, the In content is more preferably not higher than 1.0%, still more preferably not higher than 0.5%, and yet more preferably lower than 0.3%.

On the other hand, if In is to precipitate as oxides, it is preferable to increase the In content compared with cases where In precipitates as metal In. If In is to precipitate as oxides, a lower limit for In content is preferably 0.3%, more preferably 0.6%, still more preferably 0.8%, and yet more preferably 1.0%.

The other chemical components of the steel material for sliding members according to the present embodiment are not limited to any particular ones. The steel material for sliding members according to the present embodiment may be any of steel materials of various chemical compositions, depending on the application in which the steel is to be used. Non-limiting examples of the chemical composition of the steel material for sliding members according to the present embodiment will be given below.

C: 0.05 to 1.80%

Carbon (C) increases the hardenability of steel and contributes to improvements in hardness. If C content is lower than 0.05%, the hardenability of steel may be insufficient. On the other hand, if C content exceeds 1.80%, the rollability and workability of the steel may be low. In view of this, C content is preferably 0.05 to 1.80%. A lower limit for C content is more preferably 0.15%, and still more preferably 0.25%. In implementations where the strength of steel is important, a lower limit for C content is more preferably 0.30%, still more preferably 0.35%, and yet more preferably 0.40%. On the other hand, in implementations where the rollability and workability of steel are important, C content is preferably not higher than 1.50%; further, in implementations where machinability is important, C content is preferably not higher than 1.00%. In implementations where machinability is important, an upper limit for C content is more preferably 0.60%, still more preferably 0.55%, and yet more preferably 0.50%.

Si: Up to 1.5%

Silicon (Si) is used as a deoxidizer for steel. However, if Si content exceeds 1.5%, the heat conductivity of steel is low such that a sufficient seizure resistance may not be obtained. In view of this, Si content is preferably not higher than 1.5%. An upper limit for Si content is more preferably 1.0%, still more preferably 0.80%, yet more preferably 0.70%, and still more preferably 0.50%. On the other hand, Si may be intentionally included in steel, since it has the effect of preventing production of coarse cementite. To ensure that this effect is significant, Si content is preferably not lower than 0.05%, and more preferably not lower than 0.10%.

Mn: Up to 2.0%

Manganese (Mn) has the effect of increasing the hardenability of steel. However, if Mn content exceeds 2.0%, the heat conductivity of steel is low such that a sufficient seizure resistance may not be obtained. In view of this, Mn content is preferably not higher than 2.0%. An upper limit for Mn content is more preferably 1.8%, still more preferably 1.6%, yet more preferably 1.5%, still more preferably 1.0%, and yet more preferably 0.5%. On the other hand, Mn may be intentionally included in steel, since it has the effect of improving hardenability and, in addition, preventing recovery of dislocations. To ensure that these effects are significant, Mn content is preferably not lower than 0.05%, still more preferably not lower than 0.10%, and yet more preferably not lower than 0.20%.

P: Up to 0.10%

Phosphorous (P) is contained as impurities in steel. If P content exceeds 0.10%, excessive P segregates along the grain boundaries, potentially decreasing the fatigue strength of the steel. In view of this, P content is preferably not higher than 0.10%. An upper limit for P content is more preferably 0.08%, still more preferably 0.06%, yet more preferably 0.05%, and still more preferably 0.03%.

S: Up to 0.10%

Sulfur (S) is contained as impurities in steel. If S content exceeds 0.10%, hot workability may be low. In view of this, S content is preferably not higher than 0.10%. An upper limit for S content is more preferably 0.080%, still more preferably 0.070%, yet more preferably 0.050%. On the other hand, S may be intentionally included in steel to form sulfide-based inclusions, thus improving the machinability of the steel. To ensure that this effect is significant, S is preferably not lower than 0.005%, and more preferably not lower than 0.010%.

Al: Up to 0.10%

Aluminum (Al) is included in steel to serve as a deoxidizer. If Al content exceeds 0.10%, the machinability of the steel may be low. In view of this, Al content is preferably not higher than 0.10%. On the other hand, Al may be intentionally included in steel, since the resulting nitrides have the pinning effect, contributing to making austenite grains finer. To ensure that this effect is significant, Al content is preferably not lower than 0.005%, more preferably not lower than 0.010%, still more preferably not lower than 0.020%. An upper limit for Al content is more preferably 0.080%, still more preferably 0.060%, yet more preferably 0.055%, and still more preferably 0.050%.

N: Up to 0.030%

Nitrogen (N) is contained as impurities in steel. If N content exceeds 0.030%, the toughness of the steel may be low. In view of this, N content is preferably not higher than 0.030%. On the other hand, N may be intentionally included in steel, since the resulting nitrides have the pining effect, contributing to making austenite grains finer. To ensure that this effect is significant, N content is preferably not lower than 0.001%, still more preferably not lower than 0.0015%, yet more preferably not lower than 0.002%. An upper limit for N content is more preferably 0.020%, still more preferably 0.015%.

In addition to the above-discussed elements, the chemical composition of the steel may further include one or more elements selected from those discussed below. The reasons for limitations regarding each of these elements will given below.

Cr: 0 to 15.0%

Chromium (Cr) has the effect of improving strength and wear resistance. In addition, Cr is effective to prevent coarsening of austenitic structure. Accordingly, Cr may be included as necessary. However, if Cr content exceeds 15.0%, the resulting strength and toughness may be unbalanced. In view of this, Cr content is preferably not higher than 15.0%. An upper limit for Cr content is more preferably 10.0%, and still more preferably 5.0%. A lower limit for Cr content is preferably 0.01%, more preferably 0.02%, still more preferably 0.05%, and yet more preferably 0.10%. In implementations where machinability is important, Cr content is preferably not higher than 0.30%. In implementations where machinability is important, Cr content is more preferably not higher than 0.25%, and still more preferably not higher than 0.20%.

Ni: 0 to 0.50%

Nickel (Ni) has the effect of improving the strength and toughness of steel. Accordingly, Ni may be included as necessary. However, if Ni content exceeds 0.50%, saturation is reached in terms of its effects, and an increase in alloy costs results. In view of this, Ni content is preferably not higher than 0.50%. An upper limit for Ni content is more preferably 0.40%, and still more preferably 0.35%. If Ni is not intentionally included in steel, Ni content is preferably not higher than 0.10%, and more preferably not higher than 0.05%.

Cu: 0 to 0.50%

Copper (Cu) has the effect of improving the strength and toughness of steel. Accordingly, Cu may be included as necessary. However, if Cu content exceeds 0.50%, saturation is reached in terms of its effects, and an increase in alloy costs results. In view of this, Cu content is preferably not higher than 0.50%. An upper limit for Cu content is more preferably 0.40%, and still more preferably 0.35%. If Cu is not intentionally included in steel, Cu content is preferably not higher than 0.10%, and more preferably not higher than 0.05%.

Ti: 0 to 0.050%

Titanium (Ti) forms nitrides and carbonitrides, and exhibits the pinning effect which contributes to making austenite grains finer. Accordingly, Ti may be included as necessary. However, if Ti content exceeds 0.050%, the resulting toughness of the steel may be low. In view of this, Ti content is preferably not higher than 0.050%. An upper limit for Ti content is more preferably 0.040%, and still more preferably 0.030%. A lower limit for Ti content is preferably 0.005%, and more preferably 0.010%.

Nb: 0 to 0.050%

Niobium (Nb) forms nitrides and carbonitrides, and exhibits the pinning effect which contributes to making austenite grains finer. Accordingly, Nb may be included as necessary. However, if Nb content exceeds 0.050%, the resulting toughness of the steel may be low. In view of this, Nb content is preferably not higher than 0.050%. An upper limit for Nb content is more preferably 0.040%, and still more preferably 0.030%. A lower limit for Nb content is preferably 0.005%, and more preferably 0.010%.

V: 0 to 2.5%

Vanadium (V) forms nitrides and carbonitrides, and exhibits the pinning effect which contributes to making austenite grains finer. Further, it improves the strength of the steel by forming carbides. Accordingly, V may be included as necessary. However, if V content exceeds 2.5%, the resulting toughness of the steel may be low. In view of this, V content is preferably not higher than 2.5%. An upper limit for V content is more preferably 2.0%, still more preferably 1.5%, and particularly preferably 1.0%. A lower limit for V content is preferably 0.005%, and more preferably 0.010%.

Mo: 0 to 3.0%

Molybdenum (Mo) has the effect of increasing the hardenability of steel to improve the strength of the steel. Accordingly, Mo may be included as necessary. However, if Mo content exceeds 3.0%, the machinability of the steel may be low. In view of this, Mo content is preferably not higher than 3.0%. An upper limit for Mo content is more preferably 2.5%, still more preferably 2.0%, and particularly preferably 1.5%. If Mo is intentionally included in steel, a lower limit for Mo content is preferably 0.3%, and more preferably 0.5%. If Mo is not intentionally included, Mo content is preferably not higher than 0.10%, and more preferably not higher than 0.05%.

W: 0 to 6.0%

Similarly to Mo, tungsten (W) has the effect of increasing the hardenability of steel to improve the strength of steel. Accordingly, W may be included as necessary. However, if W content exceeds 6.0%, the machinability of steel may be low. In view of this, W content is preferably not higher than 6.0%. An upper limit for W content is more preferably 4.0%, and still more preferably 2.0%. A lower limit for W content is preferably 0.01%, still more preferably 0.1%, and yet more preferably 0.5%.

B: 0 to 0.005%

Boron (B) serves as a boundary-strengthening element that contributes to improvements in toughness. Accordingly, B may be included as necessary. However, if B content exceeds 0.005%, the resulting toughness may be low, rather than high. In view of this, B content is preferably not higher than 0.005%. An upper limit for B content is more preferably 0.004%, and still more preferably 0.002%. A lower limit for B content is preferably 0.0003%, and more preferably 0.0005%. To sufficiently produce the effects of B, it is preferable that N in steel is fixed by Ti.

In the chemical composition of the steel, the balance is Fe and impurities. As used herein, “impurity” means an ingredient of industrially produced steel originating from raw material such as ore or scrap or an ingredient that has entered the steel for various reasons during the manufacture process and that is within an acceptable range and thus does not adversely affect the present invention.

Elements that can be present in steel and are regarded as impurities include, for example, Pb, Ca, Mg, Sb, Ta and REMs. Even if the steel contains some or all of these elements, the present invention can be carried out without a problem if Pb content is not higher than 0.10%, Ca content not higher than 0.001%, Mg content not higher than 0.001%, Sb content not higher than 0.005%, Ta content not higher than 0.10%, and REM content not higher than 0.001%.

The following five compositions are typical examples of the composition of the steel described above.

(a) Steel containing 0.35 to 0.60% C, up to 0.50% Si, up to 0.80% Mn, up to 0.10% P, up to 0.050% S, 0.005 to 0.060% Al, 0.001 to 0.020% N, 0.05 to 5.0% In, 0 to 0.30% Cr, 0 to 0.20% Ni, 0 to 0.10% Cu, 0 to 0.050% Nb, 0 to 3.0% Mo, and the balance Fe and impurities.

(b) Steel containing 0.35 to 0.40% C, up to 0.80% Si, 1.00 to 1.80% Mn, up to 0.10% P, up to 0.070% S, 0.005 to 0.060% Al, 0.001 to 0.020% N, 0.05 to 5.0% In, 0 to 0.25% Cr, 0 to 0.15% Ni, 0 to 0.25% Cu, 0 to 0.050% Ti, 0 to 0.050% Nb, 0 to 0.10% Mo, and the balance Fe and impurities.

(c) Steel containing 0.95 to 1.10% C, 0.15 to 0.30% Si, below 0.40% Mn, up to 0.020% P, below 0.020% S, 0.005 to 0.060% Al, 0.001 to 0.020% N, 0.05 to 5.0% In, 1.30% to 1.60% Cr, 0 to 0.15% Ni, below 0.20% Cu, 0 to 0.050% Ti, 0 to 0.050% Nb, 0 to 2.5% V, 0 to 3.0% Mo, 0 to 0.005% B, and the balance Fe and impurities.

(d) Steel containing 1.40 to 1.60% C, below 0.40% Si, below 0.60% Mn, up to 0.020% P, below 0.020% S, 0.005 to 0.060% Al, 0.001 to 0.030% N, 0.05 to 5.0% In, 11.0 to 13.0% Cr, 0 to 0.50% Ni, below 0.40% Cu, 0 to 0.050% Ti, 0 to 0.050% Nb, 0.20 to 0.50% V, 0.80 to 1.20% Mo, 0 to 0.005% B, and the balance Fe and impurities.

(e) Steel containing 0.05 to 0.60% C, up to 1.5% Si, up to 2.0% Mn, up to 0.3% Cr, 0.05 to 5.0% In, up to 0.10% P, up to 0.10% S, up to 0.10% Al, up to 0.10% Cu, up to 0.10% Ni, up to 0.10% Mo, up to 0.03% N, and the balance Fe and impurities.

[Microstructure]

The steel material for sliding members according to the present embodiment has a microstructure having metal In particles or particles of oxides mainly composed of In dispersed therein.

The solid solubility limit for In in the Fe—In binary system is approximately 0.57%; however, since various other elements are dissolved in steel, the solid solubility limit for In in steel is lower than the solid solubility limit in Fe—In. Indium that has not dissolved in steel is present as precipitates. This undissolved In is diffused across at the surface of the steel even in a normal-temperature environment and forms an In concentration layer, and this In concentration layer serves as a lubrication coating to contribute to improvements in sliding properties. Further, when the lubrication coating wears out due to sliding, undissolved In in the steel diffuses across the steel surface without delay to reproduce a lubrication coating.

To ensure that the lubrication coating is reproduced more quickly to keep good sliding properties for a prolonged period of time, fine metal In particles or particles of oxides mainly composed of In and must be dispersed, and the number density of In particles required differs depending on the form of In precipitates. Specifically, good sliding properties can be obtained for a prolonged period of time if the number density of particles of metal In with diameters of 50 nm to 5 μm is not lower than 5000 particles/mm² or the density of particles of oxides mainly composed of In with diameters not smaller than 1 μm is not lower than 30 particles/mm². As used herein, “diameter”, for non-spherical particles, means equivalent circle diameter as measured in a cut surface being observed.

The microstructure of the steel material for sliding members is only required to satisfy one of the following conditions: (a) the number density of those ones of the metal In particles which have diameters of 50 nm to 5 μm is not lower than 5000 particles/mm²; and (b) the number density of those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm is not lower than 30 particles/mm². That is, if the number density of those ones of the metal In particles which have diameters of 50 nm to 5 μm is not lower than 5000 particles/mm², the number density of the particles of oxides mainly composed of In can take any value. Further, if the number density of those ones of the particles of oxides mainly composed of In that have diameters not smaller than 1 μm is not lower than 30 particles/mm², the number density of the metal In particles can take any value.

The matrix of the microstructure of the steel material for sliding members is not limited to any particular one. The matrix of the microstructure of the steel material for sliding members may be, for example, martensite, tempered martensite, ferrite-pearlite, or pearlite.

[Metal In]

To provide desired sliding properties by means of metal In, the number density of metal In particles with diameters of 50 nm to 5 μm is to be not lower than 5000 particles/mm². The number density of metal In particles with diameters of 50 nm to 5 μm is preferably not lower than 10000 particles/mm², more preferably not lower than 15000 particles/mm², and still more preferably not lower than 20000 particles/mm².

As used herein, “metal In” means In that does not form a compound such as an oxide but is single-precipitated.

The number density of metal In particles is measured in the following manner: A test specimen is taken from a region of the steel material near its surface, where the cut surface perpendicular to that surface is treated as the surface to be observed. The surface to be observed is mirror-polished and, thereafter, an electron probe microanalyzer (EPMA) is used with a magnification of 5000 to produce a backscattered electron image of the composition, which is then observed; further, a wavelength dispersive spectrometer (WDS) is used to produce an In mapping image, from which metal In is identified in various approaches, and the number of particles with diameters of 50 nm to 5 μm is counted. The number of particles divided by the area of the field of view is treated as number density.

The diameter of metal In particles being counted here is limited to the range of 50 nm to 5 μm for convenience in measurement. That is, particles smaller than 50 nm are difficult to distinguish from noise, whereas particles larger than 5 μm expand beyond the field of observation, making correct counting impossible. It suffices if the microstructure of the steel material for sliding members have metal In particles with diameters of 50 nm to 5 μm in a number density not lower than 5000 particles/mm²; it is permissible if metal In particles smaller than 50 nm or larger than 5 μm are additionally present.

The number density of metal In particles can be regulated by adjusting the In content in the steel material and quenching conditions. Specifically, there is a tendency that the higher the In content in the steel material, the higher the number density of metal In particles. On the other hand, there is a tendency that the higher the holding temperature for quenching, or the longer the holding time for quenching, the lower the number density of metal In particles.

Preferably, the steel material for sliding members according to the present embodiment includes an In concentration layer in a surface-layer portion. In concentration layer is defined as a layer with an In concentration not lower than 10 at % as measured by Auger electron spectroscopy. In the steel material for sliding members according to the present embodiment, this In concentration layer works as a lubrication coating, which provides good sliding properties. The In concentration layer preferably has a thickness not smaller than 3 nm, and more preferably not smaller than 5 nm.

The presence/absence and thickness of an In concentration layer can be determined or measured by Auger electron spectroscopy. Specifically, element analysis is repeated while Ar sputtering is being performed beginning at the surface of the steel material, to determine or measure the presence/absence and thickness of an In concentration layer. The depth for analysis is calculated based on a test for which the standard specimen is made of SiO₂.

[Oxides Mainly Composed of In]

If sliding properties are to be provided by oxides mainly composed of In, good sliding properties for a prolonged period of time is provided if the number density of particles of oxides mainly composed of In having diameters not smaller than 1 μm is not lower than 30 particles/mm². The number density of particles of oxides mainly composed of In having diameters not smaller than 1 μm is preferably not lower than 50 particles/mm², and more preferably not lower than 100 particles/mm².

As used herein, “oxides mainly composed of In” means oxides with an In content relative to cations not lower than 50% in atom %.

The number density of particles of oxides mainly composed of In is measured in the following manner: A specimen is mirror-polished and installed in an Auger electron spectroscope (AES) before the surface of the specimen is subjected to Ar-ion sputtering. Then, the surface directly after sputtering is analyzed by SEM-EDS equipment. In-kα beams and O-kα beams are separately detected, and particles in which both indium and oxygen are present are extracted by mapping image processing, and these particles are treated as oxides mainly composed of In. The above-mentioned SEM-EDS analysis is performed with an observation magnification of 100, where the number of those particles of oxides mainly composed of In which have equivalent circle diameters not smaller than 1 μm is counted, and the total of such counts is treated as number of particles. The number of particles divided by the area of the field of view is treated as number density.

If the steel contains a large amount of In, this may lead to brittle fracture and corrosion along the grain boundaries. Thus, it is desirable to increase the number density of particles of such oxides as defined above, while keeping the In content in the steel as low as possible. Specifically, it is preferable that the number density of such oxides, relative to the In content, satisfies the expression below, (i). Performing the special heat treatment discussed below promotes precipitation of such oxides, making it possible to satisfy expression (i).

M>80×In  (i)

In this expression, M indicates the number density of particles of oxides mainly composed of In contained in the steel (particles/mm²), and In indicates the In content in the steel (in mass %).

[Method of Manufacturing Steel Material for Sliding Members]

Now, an exemplary method of manufacturing the steel material for sliding members according to the present invention will be described. The method described below is merely an example, and the method of manufacturing the steel material for sliding members according to the present embodiment is not limited thereto.

Steel containing 0.05 to 5.0% In is smelt and hot forged to produce raw material. The raw material may subjected to hot working and/or cold working, as necessary. If oxides mainly composed of In are to be dispersed, the In content in the raw material is preferably 0.3 to 5.0%.

A method with metal In particles to be dispersed and a method with oxides mainly composed of In to be dispersed will be separately discussed below.

If metal In particles are to be dispersed, the raw material is subjected to quenching. The quenching may be performed by, for example, heating at 800 to 1200° C. for 5 to 30 minutes and then water cooling or oil cooling.

There is a tendency that the higher the holding temperature for quenching or the longer the holding time, the lower the number density of metal In particles. In view of this, the holding temperature for the quenching is preferably not higher than 1100° C., and more preferably not higher than 1050° C. The holding time for the quenching is preferably not longer than 20 minutes, and more preferably not longer than 15 minutes.

After the quenching, a tempering process may be performed as necessary, in which heating at 150 to 650° C. for 5 to 60 minutes is followed by air cooling or water cooling. In cases with this tempering, if the holding temperature for the tempering is excessively high or the holding time for the tempering is excessively long or the holding is followed by slow cooling (for example furnace cooling), then, indium oxides are formed due to supply of oxygen through the surface layer, potentially decreasing the number density of metal In particles. The cooling rate at this stage is preferably not lower than 1° C./s. The holding temperature for the tempering is preferably not higher than 500° C., and more preferably not higher than 450° C. The holding time for the tempering is preferably not longer than 30 minutes, and more preferably not longer than 20 minutes.

If oxides mainly composed of In are to be dispersed, the raw material is first subjected to quenching. The quenching may be performed by, for example, heating at 800 to 1200° C. for 5 to 30 minutes before water cooling or oil cooling. The holding temperature for the quenching is preferably 850 to 1050° C. The holding time for the quenching is preferably not longer than 20 minutes, and more preferably not longer than 15 minutes.

If oxides mainly composed of In are to be dispersed, the quenching is followed by a heat treatment in which the material is heated at 150 to 650° C. for 5 to 60 minutes and then subjected to furnace cooling. Thus, oxides mainly composed of In are formed during cooling. The cooling rate at this stage is preferably not higher than 2° C./min.

Further, to increase the number density of such precipitates, it is preferable to further perform a special heat treatment. The special heat treatment may be performed by a heat treatment in which, for example, the material is heated at 1000 to 1200° C. for 5 to 30 minutes and then gradually cooled at an average cooling rate not higher than 300° C./h.

The steel material for sliding members according to an embodiment of the present invention and an exemplary method of manufacturing the material have been described. The present embodiment provides a steel material for sliding members with improved seizure resistance.

EXAMPLES

Now, the present invention will be described more specifically by means of examples. The present invention is not limited to these examples.

Example 1

A steel having the chemical composition shown in Table 1 was smelted. Steel types A to I in Table 1 are based on the S45C carbon steel for mechanical structures in JIS G 4051, and were made by modifying such a steel in terms of the amount of In added and other factors. Steel types J to S in Table 1 are based on the SMn438 alloy steel for mechanical structures in JIS G 4052, and were made by modifying such a steel in terms of the amount of In added and other factors. “-” for a chemical component in Table 1 indicates that the relevant element was at an impurity level.

TABLE 1 Steel Chemical composition (in mass %, balance Fe and impurities) type C Si Mn Cr P S Al Cu Ni Mo N In A 0.45 0.20 0.70 0.12 0.016 0.011 below 0.001 below 0.02 0.02 below 0.02 below 0.02 — B 0.45 0.20 0.70 0.12 0.016 0.011 below 0.001 below 0.02 0.03 below 0.02 below 0.02 0.01 C 0.45 0.20 0.70 0.12 0.016 0.011 below 0.001 below 0.02 0.03 below 0.02 below 0.02 0.05 D 0.45 0.20 0.70 0.12 0.016 0.011 below 0.001 below 0.02 0.03 below 0.02 below 0.02 0.15 E 0.42 0.23 0.83 0.12 0.016 0.011 below 0.001 below 0.02 0.02 below 0.02 below 0.02 0.31 F 0.42 0.25 0.82 0.12 0.016 0.011 below 0.001 below 0.02 0.01 below 0.02 below 0.02 1.00 G 0.42 0.24 0.75 0.12 0.016 0.011 below 0.001 below 0.02 0.04 below 0.02 below 0.02 2.60 H 0.42 0.20 0.70 0.12 0.016 0.011 below 0.001 below 0.02 0.02 below 0.02 below 0.02 0.05 I 0.42 0.20 0.70 0.12 0.016 0.011 below 0.001 below 0.02 0.01 below 0.02 below 0.02 0.04 J 0.38 0.50 1.40 0.17 0.011 0.057 0.02 below 0.02 0.01 below 0.02 below 0.02 — K 0.38 0.50 1.40 0.17 0.011 0.057 0.02 below 0.02 0.02 below 0.02 below 0.02 0.01 L 0.38 0.50 1.40 0.17 0.011 0.057 0.02 below 0.02 0.03 below 0.02 below 0.02 0.05 M 0.38 0.50 1.40 0.17 0.011 0.057 0.02 below 0.02 0.03 below 0.02 below 0.02 0.16 N 0.38 0.52 1.42 0.17 0.011 0.057 0.02 below 0.02 0.02 below 0.02 below 0.02 0.31 O 0.38 0.55 1.50 0.17 0.011 0.057 0.02 below 0.02 0.01 below 0.02 below 0.02 0.95 P 0.38 0.60 1.43 0.17 0.011 0.057 0.02 below 0.02 0.04 below 0.02 below 0.02 2.50 Q 0.38 0.55 1.44 0.17 0.011 0.057 0.02 below 0.02 0.02 below 0.02 below 0.02 4.00 R 0.38 0.50 1.40 0.17 0.011 0.057 0.02 below 0.02 0.01 below 0.02 below 0.02 0.05 S 0.38 0.50 1.40 0.17 0.011 0.057 0.02 below 0.02 0.04 below 0.02 below 0.02 0.04

Each steel was heated at 1200° C. for one hour and then subjected to hot forging to produce raw material. The raw material was heated at 1200° C. for one hour and then subjected to hot rolling at 950° C. to 1000° C. to be formed to a predetermined size. Thereafter, quenching and tempering were performed under the conditions shown in Table 2. Specifically, a quenching process was performed in which the material was held at 1000° C. for 10 minutes and then water cooled, followed by a tempering process in which the material was further heated at 400° C. for 15 minutes and then air cooled, resulting in a hardness of approximately HV450. For Nos. 8 and 18, the quenching was performed by holding the material at 1200° C. for 20 minutes, instead of holding it at 1000° C. for 10 minutes. For Nos. 9 and 19, the quenching was performed by holding the material at 850° C. for 10 minutes, instead of holding it at 1000° C. for 10 minutes. Every one of the resulting test materials had a martensitic microstructure.

[Number Density of Metal In]

Test specimens for precipitate observation were taken from the test materials and the number density of metal In particles with diameters of 50 nm to 5 μm was measured in accordance with the method described in connection with the embodiment.

[Hardness Measurement]

Test specimens for hardness measurement were taken from the test materials, where, for each specimen, the surface perpendicular to the direction of rolling was treated as the surface for measurement. Vickers hardness was measured at four points, arranged in the direction of the wall thickness and spaced apart from each other by 1 mm. Vickers hardness was measured in accordance with JIS Z 2244 (2009). The testing force was 300 gf (2.942 N). The average for the four points was treated as the hardness of the relevant test material.

[Measurement for Indium Concentration Layer]

Test specimens for measurement for the In concentration layer were taken from the test materials, and the thickness of the In concentration layer was measured in accordance with the method described in connection with the embodiment. The Auger electron spectroscope used was SAM670 from ULVAC-PHI, Incorporated.

[Friction Test]

Disc-shaped test specimens were taken from the test materials, each with a diameter of 20 mm and a thickness of 3 mm, and were used to conduct ball-on-disc friction tests. The tester used was a tribometer from CSM Instruments SA.

The surface of each test specimen was roughly polished using #800 abrasive grains and, at the last stage, mirror-finished using 0.3 μm diamond slurry. The ball used was made of alumina and had a diameter of 6 mm, and friction testing was conducted with a load of 10 N, with the test temperature being room temperature, with a diameter of rotation of 6 mm, at a friction rate of 10 mm/s, for a friction time of 200 seconds, and with no lubricant. The coefficient of friction used was a value provided by the software of the tester.

Seizure resistance was evaluated based on the maximum coefficient of friction obtained from the above-described tests. “Maximum coefficient of friction” means the maximum value of coefficient of friction from the beginning to the end of friction (by a distance of sliding of 2.0 m). In cases where the maximum coefficient of friction exceeded 0.5, seizure was found in the post-test observation of sliding tracks. On the other hand, in cases where the maximum coefficient of friction was not higher than 0.5, no seizure was found in the post-test observation of sliding tracks. In view of this, a material with a maximum coefficient of friction not higher than 0.5 was determined to have good seizure resistance. FIG. 1 shows how coefficient of friction changed over time for a test material with a maximum coefficient of friction higher than 0.5 (No. 10 in Table 2, shown below) and a test material with a maximum coefficient of friction not higher than 0.5 (No. 15 in Table 2).

The period of time until coefficient of friction exceeded 0.4 was treated as “duration”, and a material with a duration of 100 seconds or longer was determined to have good durability.

The conditions for heat treatment and the evaluation results for the various test materials are shown in Table 2. A value in the column “Metal In particles” indicates the number density of metal In particles with diameters of 50 nm to 5 μm. A value in the column “In layer thickness” indicates the thickness of the In concentration layer.

TABLE 2 Metal In In layer Max. Steel Conditions for heat treatment particles Hardness thickness friction Duration No. type Quenching Tempering (mm⁻²) (HV) (nm) coeff. Seizure (s) Other info. 1 A 1000° C. × 10 min 400° C. × 15 min 0 446 0 0.80 yes 10 comp. ex. 2 B 1000° C. × 10 min 400° C. × 15 min 1,500 450 5 0.78 yes 25 comp. ex. 3 C 1000° C. × 10 min 400° C. × 15 min 7,300 455 10 0.35 no 105 inv. ex. 4 D 1000° C. × 10 min 100° C. × 15 min 24,510 454 25 0.33 no 110 5 E 1000° C. × 10 min 400° C. × 15 min 44.118 449 31 0.31 no 150 6 F 1000° C. × 10 min 400° C. × 15 min 270,000 447 72 0.35 no 140 7 G 1000° C. × 10 min 400° C. × 15 min 560,000 452 100 0.40 no 130 8 H 1200° C. × 20 min 400° C. × 15 min 3,000 442 2 0.52 yes 50 comp. ex. 9 I  850° C. × 10 min 400° C. × 15 min 6,000 449 2 0.55 yes 40 comp. ex. 10 J 1000° C. × 10 min 400° C. × 15 min 0 448 0 0.74 yes 15 comp. ex. 11 K 1000° C. × 10 min 400° C. × 15 min 900 460 1 0.75 yes 20 comp. ex. 12 L 1000° C. × 10 min 400° C. × 15 min 8,000 443 11 0.45 no 105 inv. ex. 13 M 1000° C. × 10 min 400° C. × 15 min 24,510 444 21 0.40 no 120 14 N 1000° C. × 10 min 400° C. × 15 min 44,118 450 49 0.36 no 130 15 O 1000° C. × 10 min 400° C. × 15 min 167,300 451 58 0.47 no 120 16 P 1000° C. × 10 min 400° C. × 15 min 625,000 453 69 0.37 no 140 17 Q 1000° C. × 10 min 400° C. × 15 min 800,000 448 95 0.38 no 140 18 R 1200° C. × 20 min 400° C. × 15 min 4,000 453 1 0.60 yes 30 comp. ex. 19 S  850° C. × 10 min 400° C. × 15 min 7,000 430 2 0.65 yes 20 comp. ex.

The test materials labeled Nos. 3 to 7 and 12 to 17 each had a chemical composition containing 0.05 to 5.0% In and a microstructure with metal In particles dispersed therein, where the number density of particles with diameters of 50 nm to 5 μm was not lower than 5000 particles/mm². These test materials had maximum coefficients of friction not higher than 0.5 and durations not shorter than 100 seconds.

Seizure was found in the test materials labeled Nos. 1, 2, 8 to 11, 18 and 19 after the tests. Seizure occurred in the test materials labeled 1, 2, 10 and 11 presumably because the In content was low and the number density of metal In particles was also low. Seizure occurred in the test specimens labeled Nos. 8 and 18 presumably because the number density of metal In particles was low. Seizure occurred in the test materials labeled Nos. 9 to 19 presumably because the In content was low.

Example 2

Next, the test materials produced without tempering after quenching were evaluated in the same manner.

The test materials were produced in the same manner as those for Example 1 except that the tempering was omitted. Similar to Example 1, the number density of metal In particles, hardness and In layer thickness were measured. Every one of the obtained test materials had a martensitic microstructure.

[Friction Test]

For friction testing, friction tests in a wet environment were conducted instead of friction tests in a dry environment as in Example 1. Similar to Example 1, disc-shaped test specimens were taken from the test materials, each with a diameter of 20 mm and a thickness of 3 mm, and were used to conduct ball-on-disc friction tests. The tester used was a tribometer from CSM Instruments SA.

The surface of each test specimen was roughly polished using #800 abrasive grains and, at the last stage, mirror-finished using 0.3 μm diamond slurry. The ball used was made of SUJ2 and had a diameter of 6 mm, and friction testing was conducted with a load of 10 N, at a test temperature of 140° C., with a diameter of rotation of 6 mm, at a friction rate of 0.5 m/s, for a friction time of 60 minutes, and with the lubricant being 2 ml engine oil. The engine oil used had a viscosity corresponding to 0W-8, and contained an organic molybdenum complex, zinc dialkyldithiophophate, and calcium sulfonate as additives. The coefficient of friction used was a value provided y the software of the tester.

Similar to Example 1, seizure resistance was evaluated based on the obtained maximum coefficient of friction. “Maximum coefficient of friction” means the maximum value of coefficient of friction from the beginning to the end of friction (by a distance of sliding of 1800 m). In cases where the maximum coefficient of friction exceeded 0.5, seizure was found in the post-test observation of sliding tracks. On the other hand, in cases where the maximum coefficient of friction was not higher than 0.5, no seizure was found in the post-test observation of sliding tracks. In view of this, a material with a maximum coefficient of friction not higher than 0.5 was determined to have good seizure resistance. FIG. 2 shows how coefficient of friction changed over time for a test material with a maximum coefficient of friction higher than 0.5 (No. 30 in Table 3, shown below) and a test material with a maximum coefficient of friction not higher than 0.5 (No. 35 in Table 3)

The period of time until coefficient of friction exceeded 0.4 was treated as “duration”, and a material with a duration of 60 minutes or longer was determined to have good durability.

The conditions for treatment and the evaluation results for the various test materials are shown in Table 3. A value in the column “Metal In particles” indicates the number density of metal In particles with diameters of 50 nm to 5 μm. A value in the column “In layer thickness” indicates the thickness of the In concentration layer.

TABLE 3 Metal In In layer Max. Steel Conditions for heat treatment particles Hardness thickness friction No. type Quenching Tempering (mm⁻²) (HV) (nm) coeff. Seizure Duration Other info 21 A 1000° C. × 10 mm N/A 0 643 0 0.67 yes below 60 min comp. ex. 22 B 1000° C. × 10 min 1,600 640 1 0.70 yes below 60 min comp. ex. 23 C 1000° C. × 10 min 8,000 655 3 0.20 no 60 min or above inv. ex. 24 D 1000° C. × 10 min 25,000 647 22 0.19 no 60 min or above 25 E 1000° C. × 10 min 45,000 642 44 0.20 no 60 min or above 26 F 1000° C. × 10 min 300,000 645 65 0.18 no 60 min or above 27 G 1000° C. × 10 min 600,000 647 70 0.21 no 60 min or above 28 H 1200° C. × 20 min 3,300 650 2 0.60 yes below 60 min comp. ex. 29 I 850° C. × 10 min 6,200 641 2 0.65 yes below 60 min comp. ex. 30 J 1000° C. × 10 min N/A 0 641 0 0.73 yes below 60 min comp. ex. 31 K 1000° C. × 10 min 1,100 642 1 0.64 yes below 60 min comp. ex. 32 L 1000° C. × 10 min 9,000 656 4 0.20 no 60 min or above inv. ex. 33 M 1000° C. × 10 min 26,000 639 29 0.18 no 60 min or above 34 N 1000° C. × 10 min 44,000 645 38 0.20 no 60 min or above 35 O 1000° C. × 10 min 190,000 643 56 0.23 no 60 min or above 36 P 1000° C. × 10 min 650,000 642 77 0.20 no 60 min or above 37 Q 1000° C. × 10 min 900,000 650 80 0.22 no 60 min or above 38 R 1200° C. × 20 min 4,200 644 1 0.70 yes below 60 min comp. ex. 39 S 850° C. × 10 min 7,500 639 2 0.71 yes below 60 min comp. ex.

The test materials labeled Nos. 23 to 27 and 32 to 37 each had a chemical composition containing 0.05 to 5.0% In and a microstructure with metal In particles dispersed therein, where the number density of particles with diameters of 50 nm to 5 μm was not lower than 5000 particles/mm². These test materials had a maximum coefficient of friction not higher than 0.5 and a duration not shorter than 60 minutes.

Seizure was found in the test materials labeled Nos. 21, 22, 28 to 31, 38 and 39 after the tests. Seizure occurred in the test materials labeled Nos. 21, 22, 30 and 31 presumably because the In content was low and the number density of metal In particles was low. Seizure occurred in the test specimens labeled Nos. 28 and 38 presumably because the number density of metal In particles was low. Seizure occurred in the test materials labeled Nos. 29 to 39 presumably because the In content was low.

Example 31

Steel melts having the chemical compositions shown in Table 4 were produced by adding In to a carbon steel material for mechanical structures constituted by the S45C steel in the JIS G 4051:2016 standard (Test Nos. 1-1 to 1-7).

TABLE 4 Test Chemical composition (in mass %, balance Fe and impurities) No. C Si Mn P S Al N In Cr Ni Cu Nb Mo 1-1 0.41 0.19 0.68 0.013 0.010 0.016 0.015 2.6 0.10 0.01 0.15 0.01 0.49 1-2 0.40 0.19 0.67 0.013 0.010 0.015 0.014 1.0 0.10 0.01 0.14 0.01 0.48 1-3 0.40 0.19 0.67 0.012 0.010 0.015 0.014 1.0 0.10 0.01 0.14 0.01 0.48 1-4 0.40 0.19 0.67 0.012 0.010 0.015 0.014 0.31 0.10 0.01 0.14 0.01 0.48 1-5 0.40 0.19 0.66 0.012 0.009 0.015 0.014 0.31 0.09 0.01 0.14 0.01 0.47 1-6 0.40 0.19 0.66 0.012 0.009 0.015 0.014 0.21 0.09 0.01 0.14 0.01 0.47 1-7 0.40 0.19 0.66 0.012 0.009 0.015 0.014 0.19 0.09 0.01 0.14 0.01 0.47

Thereafter, hot forging was performed, followed by a quenching process in which the material was heated at 900° C. for 15 minutes and then water cooled. Then, a heat treatment was performed in which the material was heated at 600° C. for 30 minutes and then furnace cooled, thus producing a test material (Test Nos. 1-1, 1-3, 1-5 to 1-7). For Test Nos. 1-2 and 1-4, the material was further subjected to a special heat treatment in which it was heated at 1100° C. for 10 minutes and then slowly cooled at an average cooling rate of 100° C./h, thus producing a test material. An observation by optical microscopy revealed that the test materials that had not been subjected to the special heat treatment had a martensitic structure, while the test materials that had been subjected to the special heat treatment had a ferritic-pearlitic structure.

Then, test specimens for precipitate observation were taken from the test materials; the surface of each specimen was mirror-polished and then installed in an AES (SAM670 from ULVAC-PHI, Incorporated). Thereafter, Ar-ion sputtering was performed on the mirror-polished surface. Then, the surface immediately after sputtering was analyzed to identify oxides mainly composed of In, and the number density thereof was determined.

Subsequently, each test specimen was used to conduct evaluation tests for seizure resistance and wear resistance. Specifically, friction properties were evaluated by ball-on-disc friction tests (with a tribometer from CSM Instruments SA). The test specimens used for friction tests were discs with a diameter of 15 mm and a thickness of 4 mm, and had a mirror-finished surface for evaluation.

Further, the ball used was made of alumina and had a diameter of 6 mm, and friction testing was conducted with a load of 10 N, with the test temperature being room temperature, with a diameter of rotation of 7 mm, at a friction rate of 10 mm/s, for a friction time of 60 minutes, and with no lubricant. The coefficient of friction used was a value provided by the software of the tester.

In the present example, seizure resistance was evaluated based on the maximum initial coefficient of friction obtained from the above-described tests. “Maximum initial coefficient of friction” in the present example means the maximum value of coefficient of friction from the beginning of friction up to a point where a distance of sliding of 1.5 m was reached.

In cases where the maximum initial coefficient of friction exceeded 0.5, seizure was found in the post-test observation of sliding tracks. On the other hand, in cases where the maximum initial coefficient of friction was not higher than 0.5, no seizure was found in the post-test observation of sliding tracks. In view of this, in the present example, a material with a maximum initial coefficient of friction not higher than 0.5 was determined to have good seizure resistance.

Further, the post-test width of the wear tracks was measured; a material with a track width not larger than 400 μm was determined to have good wear resistance.

The results are summed up in Table 5.

TABLE 5 Oxides Steel comp. Special Number Value of Seizure resistance Wear resistance Test In content heat density right side Friction Wear track No. (mass %) treatment (particles/mm²) of exp. (i)† coeff. Seizure width (μm) Evaluation 1-1 2.6 no 144 208 0.38 no 348 good inv. ex. 1-2 1.0 yes 309 80 0.35 no 328 good 1-3 1.0 no 40 80 0.41 no 369 good 1-4 0.31 yes 35 25 0.42 no 375 good 1-5 0.31 no 8 25 0.72 yes 429 poor comp. ex. 1-6 0.21 no 8 17 0.76 yes 410 poor 1-7 0.19 no 3 15 0.81 yes 413 poor †M > 80 × In . . . (i)

The specimens labeled Test Nos. 1-1 to 1-4 in Table 5, which satisfied the requirements of the present invention, had good results in terms of seizure resistance and wear resistance. In contrast, the specimens labeled Test Nos. 1-5 to 1-7, which did not satisfy the requirements of the present invention, had poor results in terms of both seizure resistance and wear resistance. Particularly, the specimens labeled Test Nos. 1-2 and 1-4, subjected to the special heat treatment, had high number densities of particles of oxides mainly composed of In and, consequently, satisfied expression (i). As a result, those specimens had significantly better results in terms of sliding properties than Test Nos. 1-3 to 1-5, which had the same In contents.

Example 4

Steel melts having the chemical compositions shown in Table 6 were produced by adding In to a steel material based on the SMn438 alloyed steel material for mechanical structures in the JIS G 4053:2016 standard (Test Nos. 2-1 to 2-5).

TABLE 6 Test. Chemical composition (in mass %, balance Fe and impurities) No. C Si Mn P S Al N In Cr Ni Cu Ti Nb Mo 2-1 0.38 0.58 1.47 0.014 0.060 0.023 0.015 0.81 0.15 0.01 0.20 0.002 0.01 0.01 2-2 0.37 0.57 1.46 0.014 0.059 0.023 0.015 0.81 0.15 0.01 0.20 0.002 0.01 0.01 2-3 0.37 0.57 1.44 0.014 0.059 0.022 0.015 0.45 0.15 0.01 0.20 0.002 0.01 0.01 2-4 0.37 0.56 1.44 0.014 0.058 0.022 0.015 0.22 0.15 0.01 0.19 0.002 0.01 0.01 2-5 0.37 0.56 1.44 0.014 0.058 0.022 0.015 0.22 0.15 0.01 0.19 0.002 0.01 0.01

Thereafter, hot forging was performed, followed by a quenching process in which the material was heated at 900° C. for 15 minutes and then water cooled. Then, a heat treatment was performed in which the material was heated at 600° C. for 30 minutes and then furnace cooled, thus producing a test material (Test Nos. 2-1, 2-4). For Test Nos. 2-2, 2-3 and 2-5, the material was further subjected to a special heat treatment in which it was heated at 1100° C. for 10 minutes and then slowly cooled at an average cooling rate of 100° C./h, thus producing a test specimen. An observation by optical microscopy revealed that the test materials that had not been subjected to the special heat treatment had a martensitic structure, while the test materials that had been subjected to the special heat treatment had a ferritic-pearlitic structure.

Then, similar to Example 3, the number density of particles of oxides mainly composed of In was measured and evaluation tests were conducted for seizure resistance and wear resistance.

The results are summed up in Table 7.

TABLE 7 Oxides Steel comp. Special Number Value of Seizure resistance Wear resistance Test In content heat density right side Friction Wear track No. (mass %) treatment (particles/mm²) of exp. (i)† coeff. Seizure width (μm) Evaluation 2-1 0.81 no 45 65 0.40 no 348 good inv. ex. 2-2 0.81 yes 120 65 0.39 no 328 good 2-3 0.45 yes 52 36 0.41 no 370 good 2-4 0.22 no 12 18 0.72 yes 430 poor comp. ex. 2-5 0.22 yes 25 18 0.49 no 420 poor †M > 80 × In . . . (i)

The specimens labeled Test Nos. 2-1 to 2-3 in Table 7, which satisfied the requirements of the present invention, had good results in terms of seizure resistance and wear resistance. In contrast, the specimens labeled Test Nos. 2-4 and 2-5, which did not satisfy the requirements of the present invention, had poor results in terms of either seizure resistance or wear resistance, or both. Particularly, the specimens labeled Test Nos. 2-2 and 2-3, subjected to the special heat treatment, had high number densities of particles of oxides mainly composed of In and thus satisfied expression (i). As a result, in a comparison between Test Nos. 2-1 and 2-2, which had the same In content, the specimen labeled Test No. 2-2, which satisfied expression (i), had better results in terms of sliding properties.

Example 51

Steel melts having the chemical compositions shown in Table 8 were produced by adding In to a steel material for high-carbon chromium bearings constituted by the SUJ2 steel in the JIS G 4805:2008 standard (Test Nos. 3-1 to 3-4).

TABLE 8 Test Chemical composition (in mass %, balance Fe and impurities) No. C Si Mn P S Al N In Cr Ni Cu Ti Nb V Mo B 3-1 1.05 0.22 0.30 0.020 0.010 0.010 0.017 0.40 1.54 0.01 0.15 0.002 0.02 0.01 0.01 0.003 3-2 1.04 0.22 0.30 0.020 0.010 0.010 0.017 0.40 1.54 0.01 0.15 0.002 0.02 0.01 0.01 0.003 3-3 1.03 0.22 0.30 0.020 0.010 0.010 0.017 0.35 1.53 0.01 0.15 0.002 0.02 0.01 0.01 0.003 3-4 1.03 0.22 0.29 0.020 0.010 0.010 0.017 0.19 1.52 0.01 0.15 0.002 0.02 0.01 0.01 0.003

Thereafter, hot forging was performed, followed by a quenching process in which the material was heated at 900° C. for 15 minutes and then oil cooled. Then, a heat treatment was performed in which the material was heated at 150° C. for 30 minutes and then furnace cooled, thus producing a test material (Test Nos. 3-1, 3-4). For Test Nos. 3-2 and 3-3, the material was further subjected to a special heat treatment in which it was heated at 1100° C. for 10 minutes and then slowly cooled at an average cooling rate of 100° C./h, thus producing a test specimen.

Then, similar to Example 3, the number density of particles of oxides mainly composed of In was measured and evaluation tests for seizure resistance and wear resistance were conducted.

The results are summed up in Table 9.

TABLE 9 Oxides Steel comp. Special Number Value of Seizure resistance Wear resistance Test In content heat density right side Friction Wear track No. (mass %) treatment (partiales/mm²) of exp. (i)† coeff. Seizure width (μm) Evaluation 3-1 0.40 no 30 32 0.38 no 324 good inv. ex. 3-2 0.40 yes 50 32 0.34 no 328 good 3-3 0.35 yes 45 28 0.35 no 334 good 3-4 0.19 no 14 15 0.75 yes 417 poor comp. ex. †M > 80 × In . . . (i)

The specimens labeled Test Nos. 3-1 to 3-3 in Table 9, which satisfied the requirements of the present invention, had good results in terms of seizure resistance and wear resistance. In contrast, the specimen labeled Test No. 3-4, which did not satisfy the requirements of the present invention, had poor results in terms of both seizure resistance and wear resistance. Particularly, the specimens labeled Test Nos. 3-2 and 3-3, subjected to the special heat treatment, had high number densities of oxides mainly composed of In and thus satisfied expression (i). As a result, in a comparison between Test Nos. 3-1 and 3-2, which had the same In content, the specimen labeled Test No. 3-2, which satisfied expression (i), had better results in terms of sliding properties.

Although an embodiment of the present invention has been described, the above-described embodiment is merely an example that can be used to carry out the present invention. Accordingly, the present invention is not limited to the above-described embodiment, and the above-described embodiment, when carried out, can be modified as appropriate without departing from the spirit of the invention.

INDUSTRIAL APPLICABILITY

The present invention provides a steel material for sliding members with good seizure resistance and good sliding properties. As such, a steel material for sliding members according to the present invention can be suitably used as a steel material for sliding members used in transportation machines such as automobiles and ships, as well as general industrial machinery, for example. 

1. A steel material for sliding members comprising: a chemical composition including, in mass %, 0.05 to 5.0% In; and a microstructure with metal In particles or particles of oxides mainly composed of In dispersed therein, those ones of the metal In particles which have diameters of 50 nm to 5 μm being in a number density not lower than 5000 particles/mm² or those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm being in a number density not lower than 30 particles/mm².
 2. The steel material for sliding members according to claim 1, wherein the chemical composition comprises, in mass %: 0.05 to 1.80% C; up to 1.5% Si; up to 2.0% Mn; up to 0.10% P; up to 0.10% S; up to 0.10% Al; up to 0.030% N; 0.05 to 5.0% In; 0 to 15.0% Cr; 0 to 0.50% Ni; 0 to 0.50% Cu; 0 to 0.050% Ti; 0 to 0.050% Nb; 0 to 2.5% V; 0 to 3.0% Mo; 0 to 6.0% W; 0 to 0.005% B; and balance Fe and impurities.
 3. The steel material for sliding members according to claim 1, wherein those ones of the metal In particles which have diameters of 50 nm to 5 μm is in a number density not lower than 5000 particles/mm².
 4. The steel material for sliding members according to claim 3, wherein the steel material includes an In concentration layer in a surface-layer portion.
 5. A method of manufacturing the steel material for sliding members according to claim 3, comprising: preparing raw material having a chemical composition including, in mass %, 0.05 to 5.0% In; and subjecting the raw material to a quenching process in which the material is heated at 800 to 1200° C. for 5 to 30 minutes and then water cooled or oil cooled.
 6. The method of manufacturing the steel material for sliding members according to claim 5, further comprising: subjecting the quenched material to a tempering process in which the material is heated at 150 to 650° C. for 5 to 60 minutes and then air cooled or water cooled.
 7. The method of manufacturing the steel material for sliding members according to claim 5, wherein no tempering process is performed after the quenching process.
 8. The steel material for sliding members according to claim 1, wherein: the chemical composition includes an In content of, in mass %, 0.3 to 5.0%, and those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm is in a number density not lower than 30 particles/mm².
 9. The steel material for sliding members according to claim 8, wherein the number density of those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm satisfies the following expression, (i): M>80×In  (i), where M indicates the number density of those ones of the particles of oxides mainly composed of In which have diameters not smaller than 1 μm (particles/mm²), and In indicates the In content in the steel (in mass %).
 10. A method of manufacturing the steel material for sliding members according to claim 8, comprising: preparing raw material having a chemical composition including, in mass %, 0.3 to 5.0% In; subjecting the raw material to a quenching process in which the material is heated at 800 to 1200° C. for 5 to 30 minutes and then water cooled or oil cooled; and subjecting the quenched material to a heat treatment in which the material is heated at 150 to 650° C. for 5 to 60 minutes and then furnace cooled.
 11. The method of manufacturing the steel material for sliding members according to claim 10, further comprising: heating the tempered material at 1000 to 1200° C. for 5 to 30 minutes and then cooling the material at an average cooling rate not higher than 300° C./h. 